Conventionally, a linear solenoid type valve (hereinafter, referred to as a “solenoid valve”) has been used as a hydraulic oil controlling device for an automatic transmission system for a vehicle. The solenoid valve is generally configured to selectively open and close an oil passage by moving an axial direction. A variety of solenoid valves have been provided. Patent Literature 1 (JP 2012-134234 A), for example, discloses one example of such a solenoid valve.
This solenoid valve generally includes a spool valve and a linear solenoid. The spool valve is operated to move along an axial direction to control a pressure of the hydraulic oil. The linear solenoid operates the spool valve to move the axial direction.
The liner solenoid includes a resin-made cylindrical bobbin, a coil, a case, and a connector. The resin-made cylindrical bobbin has flanges at both ends of the resin-made cylindrical bobbin. The coil is formed of an insulating coated wire wound around the cylindrical bobbin. The case houses a solenoid coil portion formed of the cylindrical bobbin and the coil. The connector is used to electrically connect an external equipment.
Since the solenoid valve is used in the hydraulic oil, a coil assembly has been typically used for the linear solenoid. The coil assembly has a molding structure where the solenoid coil portion and the connector are integrally molded with molding resin.
It should be noted that the coil molding portion and the connector are formed at the same time by insert-molding (i.e., secondary molding) the solenoid coil portion and a terminal with molding resin after electrically connecting the winding start end and the winding terminal end of the coil to the terminal for the external equipment.
In a system, such as an automatic transmission system for a vehicle, which is installed in an engine room and plays a pivotal role for vehicle control, demand for downsizing and cost reduction to the system including its components has been increased year and year. Therefore, there is highly demand to components, such as a solenoid valve serving as a main component for the system, for downsizing and cost reduction.
A variety of trials had been made to achieve downsizing and cost reduction. Each time, a linear solenoid was a target because the linear solenoid required an expensive manufacturing facility (a molding device) and included a coil assembly having a large size. However, it would tend to avoid simplifying an insulating structure for the coil assembly because of safety aspects considering operating conditions where the coil assembly was immersed in the hydraulic oil during its operation.
Therefore, a measure contributing to downsizing and cost reduction of the linear solenoid by simplifying an insulating structure is required.
In view of the above, it is an objective of the present disclosure to provide a solenoid valve for hydraulic control that satisfies demand for downsizing and cost reduction.